Catalyst and process for the direct synthesis of hydrogen peroxide

ABSTRACT

A description follows of a bimetallic catalyst, obtained by dispersing in sequence and alternating the precursors of the single metal components of the catalyst on a carrier, and a process for the synthesis of hydrogen peroxide by the direct reaction of hydrogen with oxygen, in a solvent medium containing a halogenated promoter and an acid promoter, in the presence of said catalyst.

[0001] The present invention relates to a bimetallic catalyst and aprocess for the direct synthesis of hydrogen peroxide from hydrogen andoxygen which uses said catalyst.

[0002] Hydrogen peroxide is a commercially important compound which iswidely used as bleach in the textile and paper industry, as a biocide inthe environmental field and in oxidation processes in the chemicalindustry.

[0003] Examples of these processes are those using titanium silicaliteas catalysts, such as the epoxidation of olefins (EP-100,119), theammoximation of carbonyl compounds (U.S. Pat. No. 4,794,198), theoxidation of ammonia to hydroxylamine (U.S. Pat. No. 5,320,819) and thehydroxylation of aromatic hydrocarbons (U.S. Pat. No. 4,369,783).

[0004] The industrial production of aqueous solutions of H₂O₂ by meansof a complex two-step process, is known. In this process a solution ofan anthraquinone, such as butylanthraquinone or ethylanthraquinone, inan organic medium immiscible with water is first hydrogenated and thenoxidized with air to produce H₂O₂ which is subsequently extracted inaqueous phase.

[0005] This procedure is expensive owing to the high investment costsnecessary for the complex production unit involved and the necessity ofseparating and disposing of the by-products generated during theoxidation phase and purifying and reintegrating the anthraquinonesolution before being re-used.

[0006] Processes for the direct synthesis of hydrogen peroxide from H₂and O₂ have been proposed in the art for over-coming thesedisadvantages. These processes generally use a catalytic systemconsisting of a noble metal, particularly metals of the platinum groupor their mixtures, in the form of salts or as supported metals.

[0007] For example, U.S. Pat. Nos. 4,772,458 and 4,832,938 describe aprocess for the synthesis of aqueous solutions of H₂O₂ using a catalystbased on palladium and/or platinum supported on carbon, in the presenceof hydrogen ions and bromide ions. Quantities of acids ranging from 2.5to 10 g/liter are generally required for obtaining high concentrationsof H₂O₂.

[0008] The use of high quantities of acids creates serious problemslinked to the dissolution of the active phase (metal) of the catalyst inthe reaction medium, with the consequent instability of both thecatalyst and hydrogen peroxide solution produced.

[0009] In addition, under these conditions, the solutions of H₂O₂produced are difficult to use owing to the high acid content. Theseprocesses also operate under critical conditions as concentrations of H₂higher than 5% (17% or over) are used with respect to the reactionmixture and consequently fall within the explosivity limit of H₂/O₂mixtures.

[0010] Patent application EP-492.064 describes a process for thesynthesis of hydrogen peroxide from hydrogen and oxygen which uses acatalyst based on palladium supported on a halogenated resin, inparticular a brominated styrene/divinylbenzene resin.

[0011] The reaction is carried out in water, in the presence of an acidselected from sulfuric, phosphoric or nitric acid. Operating accordingto this process however concentrations of H₂O₂ of about 0.58% areobtained.

[0012] Patent application EP-504,741 describes a process for thesynthesis of H₂O₂ from hydrogen and oxygen which uses a catalyst basedon palladium or platinum supported on an acid or superacid carrierselected from oxides of molybdenum, zirconium or tungsten.

[0013] Operating according to this process, hydrogen peroxide isobtained in concentrations of not more than 1%.

[0014] U.S. Pat. No. 5,320,921 describes a process for the synthesis ofH₂O₂ from hydrogen and oxygen which uses a catalyst based on palladiumor platinum supported on a heteropolyacid made insoluble in water. Thereaction is carried out in water in the presence of bromide ions (0.5mmoles/liter of sodium bromide). Concentrations of H₂O₂ equal to about1.1% are obtained.

[0015] These processes of the known art are consequently characterizedby a low productivity and selectivity of the reaction and by theproduction of H₂O₂ solutions which are too dilute for an economicindustrial exploitation.

[0016] Italian patent application MI 98-A-01843, filed by the Applicant,relates to a process for the direct synthesis of hydrogen peroxide usinga catalyst based on palladium and/or platinum co-impregnated on asulfonated activated carbon.

[0017] It has now been found that by carrying out the reaction betweenhydrogen and oxygen in the presence of a bimetallic catalyst prepared bydispersing in sequence and alternating the precursors of the singlemetal components of the catalyst on a carrier, it is possible toovercome the drawbacks of the known art described above. Thispreparation method also allows the activity and selectivity of thecatalyst, the concentration of hydrogen peroxide or consumption ofhydrogen, to be modulated in relation to the process requirements andavailability of raw materials, for example low cost hydrogen.

[0018] In particular, the use of these catalysts gives the followingadvantages:

[0019] (i) the possibility of using extremely low quantities of halides(<10⁻⁴ moles/l) and free acids (H⁺<10⁻² moles/l) in the reaction medium.This has beneficial effects on the stability of the catalytic system andH₂O₂ solutions obtained, thus allowing the possibility of directly usingthe above solutions in oxidation processes. The presence of only tracesof halides and acidity is not a disadvantage for direct use in the abovereactions as the possible introduction of salts or acidity in downstreamprocesses is minimized.

[0020] (ii) the production of hydrogen peroxide solutions in adequateconcentrations for direct use and economically valid in oxidationprocesses, generally ranging from 2% to 10% by weight.

[0021] (iii) the possibility of carrying out the reaction under highsafety conditions. In fact, below 4% by volume of hydrogen meansoperating well outside the explosivity range of inert H₂-O₂ mixtures.

[0022] In accordance with this, the present invention relates to abimetallic catalyst obtained by dispersing in sequence and alternatingthe precursors of the single metal components of the catalyst on acarrier, and a process for the synthesis of hydrogen peroxide by thedirect reaction of hydrogen with oxygen, in a solvent medium containinga halogenated promoter and an acid promoter, in the presence of saidcatalyst.

[0023] In particular, the catalyst used for the purposes of the presentinvention is obtained by:

[0024] (a) preparation of the solutions or suspensions of precursors ofthe single metal components of the catalytic system;

[0025] (b) dispersion of the solutions or suspensions obtained in (a) insequence on a carrier;

[0026] (c) treatment of the catalyst with a reducing agent of the metaland drying at 120-140° C. between one dispersion and another.

[0027] In the preparation of said catalyst, steps (b) and (c) can berepeated once or several times.

[0028] The metal components of the catalyst are selected from those ofthe platinum group. Palladium and platinum are preferably used.

[0029] In these catalysts the palladium is normally present in aquantity ranging from 0.01 to 5% by weight and the platinum in aquantity ranging from 0.01 to 1% by weight, with an atomic ratio betweenplatinum and palladium ranging from 0.1/99.9 to 50/50.

[0030] The palladium is preferably present in a quantity ranging from0.4 to 2% by weight and the platinum in a quantity ranging from 0.05 to0.5% by weight, with an atomic ratio between platinum and palladiumranging from 1/99 to 30/70.

[0031] The dispersion of the active components on the carrier can beeffected by means of precipitation, impregnation or adsorption startingfrom solutions of their salts or soluble complexes selected fromacetates, halides and nitrates.

[0032] The reduction of the catalyst components to the metal state canbe carried out by means of thermal and/or chemical treatment withreducing substances such as hydrogen, sodium formiate, sodium citrate,using preparative methods well known in the art.

[0033] The inert carrier can consist of activated carbon, silica,alumina, silica-alumina, zeolites and other materials well known in theart. Activated carbon is preferred for the preparation of the catalystsuseful for the invention.

[0034] Activated carbons which can be used for the purposes of theinvention are those with a low ash content and a surface area of atleast 100 m²/g, in particular those having a surface area greater than300 m²/g.

[0035] Sulfonated activated carbons described in Italian patentapplication MI 98-A-01843 can also be used for the purpose.

[0036] The carriers can be in powder, grain or pellet form, etc.

[0037] Before supporting the precursors of the single metals, theactivated carbon can be subjected to treatment such as washing withdistilled water or treatment with acids, bases or diluted oxidizingagents, for example acetic acid, hydrochloric acid, sodium carbonate andhydrogen peroxide.

[0038] In particular, it has been observed that the catalyst obtained bydispersing first the palladium and then the platinum on the carrier, ismore active, whereas that obtained by dispersing first the platinum andthen the palladium in sequence, is more selective.

[0039] The catalyst of the present invention is particularlyadvantageous in a process for the direct synthesis of hydrogen peroxidefrom hydrogen and oxygen in a solvent in the presence of a halogenatedpromoter and an acid promoter.

[0040] The catalyst is used in catalytic quantities generally rangingfrom 10⁻⁶ to 10⁻² moles of total metal contained in the catalyst perliter of reaction medium.

[0041] Advantageous results are obtained using quantities of catalystranging from 10⁻⁴ to 10⁻³ moles of total metal contained in the catalystper liter of reaction medium.

[0042] The reaction solvent can consist of water, a C₁-C₃ alcohol ortheir mixtures.

[0043] Among C₁-C₃ alcohols, methanol is preferred for the purposes ofthe invention. Among the mixtures, a mixture of methanol and water witha weight ratio ranging from 50/50 to 99.9/0.1, preferably from 90/10 to99/1, is preferred.

[0044] The acid promoter can be any substance capable of generating H⁺hydrogen ions in the liquid reaction medium and is generally selectedfrom inorganic acids such as sulfuric, phosphoric, nitric acids or fromorganic acids such as sulfonic acids. Sulfuric acid and phosphoric acidare preferred. The concentration of the acid generally ranges from 20 to1000 mg per kg of solution and preferably from 50 to 500 mg per kg ofsolution.

[0045] The halogenated promoter can be any substance capable ofgenerating halogen ions in the liquid reaction medium. Substancescapable of generating bromide ions are preferred. These substances aregenerally selected from hydrobromic acid and its salts soluble in thereaction medium, for example sodium bromide, potassium bromide, sodiumor ammonium bromate. Hydrobromic acid, sodium bromide and potassiumbromide are particularly preferred.

[0046] The concentration of halogenated promoter generally ranges from0.1 to 50 mg per kg of solution and preferably from 1 to 10 mg per kg ofsolution.

[0047] The production of hydrogen peroxide is carried out by reactingthe oxygen and hydrogen in the reaction medium in the presence of thecatalyst and promoters and in the presence of or without an inert gasselected from nitrogen, helium, argon. The gas is preferably nitrogen.

[0048] The molar ratio H₂/O₂ in the feeding ranges from ½ to {fraction(1/100)}, preferably from ⅓ to {fraction (1/15)} and the concentrationof hydrogen in the gaseous phase in contact with the liquid reactionmedium is conveniently maintained at a value lower than 4.5% molar,outside the explosivity limits of the H₂/O₂/inert gas mixture.

[0049] According to an embodiment of the process of the presentinvention, the reaction can be carried out using air instead of pureoxygen.

[0050] The reaction is typically carried out at temperatures rangingfrom −5° to 90° C., preferably from 2 to 50° C. and at a total pressurehigher than atmospheric pressure, preferably ranging from 50 to 300atmospheres.

[0051] The process according to the present invention can be carried outbatchwise, or, preferably, in continuous using a reactor suitable forthe purpose and selected from those described in the art.

[0052] Operating under the above conditions, it is possible to producehydrogen peroxide in safety conditions with a reaction productivitynormally ranging from 30 to 200 g of H₂O₂ (expressed as 100% H₂O₂) perliter of reaction medium per hour and with a molar selectivity towardsthe formation of H₂O₂, referring to the hydrogen used up, ranging from60% to 90%. The hydrogen peroxide solutions thus obtained can be useddirectly in oxidation processes which comprise the use of H₂O₂ withoutcostly intermediate processing, such as acid and solvent removaloperations.

[0053] The process of the present invention enables the reagents to betransformed into H₂O₂ with high conversions and selectivities, obtainingH₂O₂ solutions without acidity or containing only traces of acidityand/or salts.

[0054] The following examples, whose sole purpose is to describe thepresent invention in greater detail, should in no way be considered aslimiting its scope.

EXAMPLE 1

[0055] Preparation of the Catalyst

[0056] a) 8 g of activated maritime pine charcoal in powder form (Ceca2S/E) are charged into a 0.5 liter glass flask, containing 90 ml ofdemineralized water and 0.28 g of Na₂CO₃. The suspension is maintainedat room temperature (20-25° C.), under stirring, for 10 minutes.

[0057] 0.101 g of a solution of H₂PtCl₆ (8% by weight of Pt) diluted in10 ml of water are subsequently added dropwise.

[0058] The suspension is kept at room temperature for 10 minutes and isthen heated in a water bath for 10 minutes to 90° C. A solutioncontaining 0.76 g of sodium formiate in 10 ml of water is then added andthe stirring is continued at 90° C. for 2 hours.

[0059] After cooling to room temperature, the suspension is filtered andthe recovered charcoal is washed with distilled water until thechlorides have been eliminated and dried in an oven at 120° C. for 2hours.

[0060] b) the carbon thus obtained is re-suspended in a solutioncontaining 0.4 g of Na₂CO₃ in 90 ml of water and then treated followingthe procedure described in a), but using a solution of 0.8 g of Na₂PdCl₄at 10% of Pd, instead of the solution of H₂PtCl₆.

[0061] After drying at 120° C., a catalyst containing 1% of Pd and 0.1%of Pt on activated carbon is obtained.

EXAMPLE 2

[0062] The same procedure is adopted as described in example 1, butinverting the sequence of Pt-Pd deposit.

[0063] In step a) the solution Na₂PdCl₄ is used, and in step b) thesolution of H₂PtCl₆, maintaining the same quantities. A catalystcontaining 0.1% of Pt and 1% of Pd on activated carbon, is obtained.

EXAMPLE 3 Comparative

[0064] Preparation of a Co-impregnated Catalyst

[0065] 8 g of activated maritime pine charcoal in powder form (Ceca2S/E) are charged into a 0.5 liter glass flask, containing 90 ml ofdemineralized water and 0.28 g of Na₂CO₃. The suspension is maintainedat room temperature (20-25° C.), under stirring, for 10 minutes.

[0066] 0.101 g of a solution of H₂PtCl₆ (8% by weight of Pt) and 0.80 gof Na₂PdCl₄ (10% Pd) diluted in 10 ml of water, are subsequently addeddropwise, under stirring.

[0067] The suspension is kept at room temperature for 10 minutes and isthen heated in a water bath for 10 minutes to 90° C. A solutioncontaining 760 mg of sodium formiate in 10 ml of water is then added andthe stirring is continued at 90° C. for 2 hours.

[0068] After cooling to room temperature, the suspension is filtered andthe recovered charcoal is washed with distilled water until thechlorides have been eliminated and dried in an oven at 120° C. for 2hours.

[0069] The end catalyst contains 1% of Pd and 0.1% of Pt, the activephase was obtained by co-impregnation of Pd and Pt.

EXAMPLE 4 Comparative

[0070] The same procedure is adopted as described in example 3, butusing only the Pd salt in the preparation.

[0071] A catalyst is obtained, containing 1% of Pd on activated carbon.

EXAMPLE 5 Comparative

[0072] The same procedure is adopted as described in example 3, butusing only the Pt salt in the preparation.

[0073] A catalyst is obtained, containing 0.1% of Pt on activatedcarbon.

EXAMPLE 6

[0074] Synthesis of H₂O₂

[0075] A micropilot plant is used, consisting of a Hastelloy C autoclaveequipped with a thermostat-regulation system, magnetic drag stirring, aregulation and control system of the pressure during the reaction, afilter for continuously removing the liquid phase containing thereaction products, a feeding system of the solvent mixture in which thereaction takes place and a series of instruments for the regulation andcontrol of the gaseous feeds. The reaction trend is followed bycontinuously analyzing the hydrogen and oxygen in the feeding and at theoutlet of the reactor.

[0076] The selectivity with respect to the converted hydrogen iscalculated on the basis of the concentration of H₂O₂ in the reactioneffluent and on the basis of analysis of the H₂ leaving the reactor. Theconcentration of H₂O₂ which is formed is determined by titration withpotassium permanganate.

[0077] 0.6 g of catalyst prepared as described in example 1 and 200 g ofmethanol:water solution (95/5 by weight) containing 6 ppm of HBr and 300ppm of H₂SO₄ are charged into the reactor.

[0078] The autoclave is pressurized, without stirring, at 100 bars witha gaseous mixture consisting of 3.6% of H₂, 10% of O₂ and 86.4% of N₂.The stirring is then started up to 800 revs/minute, the pressure ismaintained with a continuous stream, 700 normal liters (Nl), of the samegaseous mixture, and 300 g/hour of a methanol:water solution having thecomposition defined above, is fed at the same time. The temperatureinside the reactor is kept at 6° C. The results are indicated inTable 1. TABLE 1 Hrs of H₂O₂ Molar selectivity reaction wt % H₂O₂ %  54.7 83 10 4.6 84 20 4.6 84 50 4.5 86

EXAMPLE 7

[0079] Example 6 is repeated using the catalyst prepared in example 2.

[0080] The results obtained are indicated in Table 2. TABLE 2 Hrs ofH₂O₂ Molar selectivity reaction wt % H₂O₂ %  5 6.5 70 10 6.6 71 20 6.471 50 6.6 72

EXAMPLE 8

[0081] Example 6 is repeated using a catalyst prepared as in example 2,but using a sulfonated carbon as carrier, prepared as described inexample 1 of Italian patent MI 98-A-01843.

[0082] The results are indicated in Table 3. TABLE 3 Hrs of H₂O₂ Molarselectivity reaction wt % H₂O₂ %  5 6.2 73  10 6.4 74  20 6.5 73  30 6.375  50 6.6 76 100 6.5 75

EXAMPLE 9 Comparative

[0083] Example 6 is repeated using the catalyst prepared in example 3.The results obtained are indicated in Table 4. TABLE 4 Hrs of H₂O₂ Molarselectivity reaction wt % H₂O₂ %  5 4.8 75 10 4.9 76 20 4.7 77 50 4.6 77

EXAMPLE 10 Comparative

[0084] Example 6 is repeated using the catalyst prepared in example 4.

[0085] The results obtained are indicated in Table 5. TABLE 5 Hrs ofH₂O₂ Molar selectivity reaction wt % H₂O₂ %  5 0.9 35 10 0.8 37

EXAMPLE 11 Comparative

[0086] Example 6 is repeated using the catalyst prepared in example 5.

[0087] The results obtained are indicated in Table 6. TABLE 6 Hrs ofH₂O₂ Molar selectivity reaction wt % H₂O₂ %  5 1.7 22 10 1.5 25

1. A process for the synthesis of hydrogen peroxide by means of thedirect reaction of hydrogen with oxygen, in a solvent medium containinga halogenated promoter and an acid promoter, in the presence of abimetallic catalyst obtained by dispersing in sequence and alternatingthe single metal components of the catalyst on a carrier.
 2. The processaccording to claim 1, wherein the metal components of the catalyst areselected from those of the platinum group.
 3. The process according toclaim 2, wherein the metals are palladium and platinum.
 4. The processaccording to claim 3, wherein the catalyst contains a quantity ofpalladium ranging from 0.01 to 5% by weight and a quantity of platinumranging from 0.01 to 1% by weight, with an atomic ratioplatinum/palladium ranging from 0.1/99.9 to 50/50.
 5. The processaccording to claim 4, wherein the catalyst contains a quantity ofpalladium ranging from 0.4 to 2% by weight and a quantity of platinumranging from 0.05 to 0.5% by weight, with an atomic ratioplatinum/palladium ranging from 1/99 to 30/70.
 6. The process accordingto claim 1, wherein the carrier is selected from activated carbon,activated carbon functionalized with sulfonic groups, silica, alumina,silica-alumina and zeolites.
 7. The process according to claim 6,wherein the carrier is an activated carbon with a low ash content and asurface area of at least 100 m²/g.
 8. The process according to claim 7,wherein the activated carbon has a surface area higher than 300 m²/g. 9.The process according to claim 1, wherein the bimetallic catalyst isobtained by: (a) preparation of the solutions or suspensions of theprecursors of the single metal components of the catalytic system; (b)dispersion of the solutions or suspensions obtained in (a) in sequenceon a carrier; (c) treatment of the catalyst with a reducing agent of themetal and drying at 120-140° C. between one dispersion and another. 10.The process according to claim 9, wherein steps (b) and (c) can berepeated once or several times.
 11. The process according to claim 9,wherein the precursors of the metals are their salts or solublecomplexes selected from acetates, halides and nitrates.
 12. The processaccording to claim 9, wherein the dispersion of the precursors of thesingle metal components of the catalytic system on the carrier iscarried out by means of precipitation, impregnation or adsorption. 13.The process according to claim 9, wherein the reduction of the metals iscarried out by means of thermal and/or chemical treatment with reducingsubstances such as hydrogen, sodium formiate or sodium citrate.
 14. Theprocess according to claim 1, wherein the reaction medium is selectedfrom water, a C₁-C₃ alcohol or a mixture of these.
 15. The processaccording to claim 14, wherein the alcohol is methanol.
 16. The processaccording to claim 14, wherein the reaction medium is a mixture ofalcohol:water with a weight ratio between the two ranging from 50:50 to99.9:0.1.
 17. The process according to claim 16, wherein the weightratio alcohol:water ranges from 90:10 to 99:1.
 18. The process accordingto claim 1, wherein the halogenated promoter is selected from substancescapable of generating halogen ions in the liquid reaction medium. 19.The process according to claim 18, wherein the halogenated promoter isselected from substances capable of generating bromide ions such ashydrobromic acid and its salts soluble in the reaction medium such asalkaline bromides, sodium bromate or ammonium bromide.
 20. The processaccording to claim 19, wherein the compound is hydrobromic acid, sodiumbromide or potassium bromide.
 21. The process according to claim 1,wherein the concentration of halogenated promoter ranges from 0.1 to 50mg per kg of solution.
 22. The process according to claim 21, whereinthe concentration of halogenated promoter ranges from 1 to 10 mg per kgof solution.
 23. The process according to claim 1, wherein the acidpromoter is selected from substances capable of generating H⁺ hydrogenions in the reaction medium.
 24. The process according to claim 23,wherein the acid promoter is selected from inorganic acids such assulfuric, phosphoric, nitric acid or from organic acids such as sulfonicacids.
 25. The process according to claim 24, wherein the acid promoteris sulfuric acid or phosphoric acid.
 26. The process according to claim1, wherein the concentration of the acid promoter ranges from 20 to 1000mg per kg of solution.
 27. The process according to claim 26, whereinthe concentration of the acid promoter ranges from 50 to 500 mg per kgof solution.
 28. The process according to claim 1, wherein the catalystis used in quantities ranging from 10⁻⁶ to 10⁻² moles of total metalcontained in the catalyst per liter of reaction medium.
 29. The processaccording to claim 28, wherein the catalyst is used in quantitiesranging from 10⁻⁴ to 10⁻³ moles of total metal contained in the catalystper liter of reaction medium.
 30. The process according to claim 1,wherein the reaction is carried out at a temperature ranging from −5 to90° C.
 31. The process according to claim 30, wherein the temperatureranges from 2 to 50° C.
 32. The process according to claim 1, whereinthe reaction is carried out at a total pressure higher than atmosphericpressure.
 33. The process according to claim 32, wherein the totalpressure ranges from 50 to 300 atmospheres.
 34. The process according toclaim 1, wherein the molar ratio hydrogen:oxygen in the feeding rangesfrom 1:2 to 1:100.
 35. The process according to claim 34, wherein themolar ratio hydrogen:oxygen in the feeding ranges from 1:3 to 1:15. 36.The process according to claim 1, wherein the reaction is carried out inthe presence of an inert gas selected from nitrogen, helium, argon. 37.The process according to claim 36, wherein the inert gas is nitrogen.38. The process according to claim 1, wherein the concentration ofhydrogen in the gaseous phase in contact with the liquid reaction mediumis maintained at a value lower than 4.5% molar.
 39. The processaccording to claim 1, wherein the reaction is carried out using air asoxygen source.
 40. The process according to claim 1, wherein thereaction is carried out batchwise or in continuous.
 41. The processaccording to claim 1, wherein the solution of hydrogen peroxide is useddirectly in an oxidation process of a substrate selected from olefins,aromatic hydrocarbons, ammonia and carbonyl compounds using titaniumsilicalite as catalyst.
 42. A bimetallic catalyst useful for theproduction of hydrogen peroxide obtained by dispersing in sequence andalternating the precursors of the single metal components of thecatalyst by : (a) preparation of the solutions or suspensions of theprecursors of the single metal components of the catalytic system; (b)dispersion of the solutions or suspensions obtained in (a) in sequenceon a carrier; (c) treatment of the catalyst with a reducing agent of themetal and drying at 120-140° C. between one dispersion and another. 43.The catalyst according to claim 42, wherein steps (b) and (c) can berepeated once or several times.
 44. The catalyst according to claim 42,wherein the metal components of the catalyst are selected from those ofthe platinum group.
 45. The catalyst according to claim 43, wherein themetals are palladium and platinum.